This invention relates generally to optical filters for optical communications networks and add-drop multiplexers and optical networking systems based on interference filters and, more particularly, to interference filters and etalon filters with temperature compensation to adjust for filter center wavelength drift over temperature.
Fiber optic cable finds widespread use in data transmission and other telecommunications applications that take advantage of the inherently large bandwidth available with fiber optics. There is a significant installed base of fiber optic cable criss-crossing much of the developed world, but the capacity of that installed base is being increasingly stretched as demand for communications network capacity surges. Fortunately, recent advances in optical signal processing have increased the number of optical signal channels that may be carried by a single fiber optic strand and have reduced the need to install expensive new fibers by greatly increasing the communications capacity of existing fibers. One of these optical signal processing advances is wavelength division multiplexing (WDM), which allows multiple optical channels, each at a different wavelength, to be carried over a single fiber optic strand. In WDM, carrier signals at different channel wavelengths are combined onto or extracted from a single strand of optical fiber capable of propagating the full range of channel wavelengths. WDM uses multiplexers and other filter-based signal processing components to provide the wavelength-selective channel combining and extracting functions.
Optical signal processing components known as interference filters are based on the principle of multiple-beam interference and can be constructed by depositing multiple thin layers of dielectric film on a silica or other substrate. Recent advances in dielectric film deposition technology and precise in-situ control of deposition have enabled the fabrication of interference filters for demanding dense WDM (DWDM) applications with narrow signal channels and close channel spacing.
Etalon filters are employed in a number of optical applications, including for wavelength locking of distributed feedback lasers and other narrow-output lasers and for adjustment of Fabry-Perot interferometers. In a laser wavelength locking application, a sample of the laser output is directed to an etalon filter tuned to the desired center wavelength of the laser. The optical energy both transmitted through and reflected from the etalon filter is measured, and the ratio of those two measurements indicates the extent to which the laser output has departed from the desired center wavelength. The ratio signal can be used in a feedback loop to control laser bias current or another laser control parameter that controls the output wavelength of the laser.
An etalon filter can be constructed by forming mirrors on each side of a dielectric spacer. The spacer can be a wafer of silicon, zinc sulfide or zinc selenide, for example. The mirrors can be formed of quarter wavelength layer pairs of dielectric films of high and low refractive index material. In operation, the center wavelength of the etalon filter may be tuned by adjusting its position relative to the angle of incidence of an incident collimated optical beam. Unfortunately, the center wavelength of an etalon filter changes with temperature, and the temperature performance of a laser output locking circuit or interferometer stabilization circuit based on an etalon filter is accordingly degraded.
In many applications it is advantageous to be able to tune the center wavelength of an interference filter or an etalon filter. For example, tuning mechanisms have been developed that take advantage of the fact that the center wavelength of an interference filter is reduced as the angle of incidence of the optical beam is moved away from the normal to the filter surface. FIG. 1 indicates that the center wavelength of a typical interference filter changes by a few percent as the angle of incidence changes from the normal through an angle of 40 or 50 degrees from the normal. Tunable interference filters that permit adjustment of the angle of incidence are disclosed in U.S. Pat. No. 5,481,402 to Cheng et al. (xe2x80x9cChengxe2x80x9d) and in European Pat. Appln. No. EP 0733921A2 to Bendelli et al. (xe2x80x9cBendellixe2x80x9d). A tunable interference filter according to Bendelli is illustrated in FIG. 2.
Newer DWDM systems combine and extract an even larger number of narrower, more closely-spaced channels onto a single fiber strand. For example, each channel may have a bandwidth of only 0.2 nm and adjacent channels may be separated by only 0.4 nm, and DWDM interference filter performance requirements are accordingly stringent. Unfortunately, interference filters in DWDM applications may have unacceptable temperature performance: as the temperature of the interference filter increases, the refractive index of the dielectric layers change and the center wavelength of the filter drifts accordingly. For the narrow channel widths and close channel spacings of DWDM systems, this drift may be unacceptable. FIG. 3 shows the dependence of the center wavelength on temperature for a typical interference filter. The temperature dependence of the center wavelength of a typical multi-layer DWDM interference filter is roughly 0.004 nm/deg C., so a temperature change of 50 deg C. will change the center wavelength of a DWDM interference filter by about 0.2 nm. For DWDM applications with channel separation as small as 0.4 nm and filter bandwidths of as narrow as 0.2 nm, such a center wavelength shift is obviously unacceptable, so a temperature compensation mechanism is needed. Similarly, the center wavelength of an etalon filter used to lock a laser or stabilize an interferometer may drift unacceptably as a function of temperature. Unfortunately, the manually tunable filters of Cheng and Bendelli do not permit compensation for temperature changes without manual intervention. Manual tuning to compensate for temperature is impractical for fiber communications equipment deployed in the field.
One conventional approach to overcoming the aforementioned problem of center wavelength or resonance wavelength drift with temperature is to actively control the temperature of the DWDM interference filter assembly or etalon filter. Active temperature control techniques are widely used in a variety of electronics applications. These techniques generally employ thermistors to measure the temperature of the device, heaters or thermoelectric coolers to control the temperature of the device, and control electronics to control the heaters or coolers based on feedback from the thermistors. Unfortunately, active temperature control schemes are bulky and expensive and consume large amounts of power.
In summary, interference filters shift over temperature and may therefore provide unacceptable performance in many DWDM applications with stringent channel selectivity requirements. Active temperature control techniques are conventionally employed to overcome these performance limitations, but an unacceptably heavy penalty is paid in cost, weight, power consumption, and reliability.
According to one aspect of the present invention, a temperature-compensated electronic equipment is provided, such as an optical filter assembly for filtering optical signals from an optical beam. The assembly compensates for temperature-induced changes in filter performance, particularly in dense wavelength division multiplexing applications characterized by narrow, closely-spaced optical signal channels. The assembly includes a frame having two spaced-apart ports for applying optical signals, an optical filter pivotally mounted to the frame between the first and second ports, and a positioner mounted to the frame and in engagement with the optical filter. The positioner is formed of a material that has a coefficient of thermal expansion different from that of the frame material. The positioner is constructed and arranged to move the optical filter in response to changes in the temperature of the frame, thereby compensating for changes in the frequency characteristics of the optical filter over temperature.
According to another aspect of the invention, a method of compensating for temperature-induced changes in the frequency characteristics of an optical filter assembly is provided. The method includes the steps of providing a frame with spaced-apart first and second optical ports, positioning an interference filter between the optical ports, and moving the interference filter to adjust the frequency characteristics of the interference filter in response to changes in the temperature of the frame.
The apparatus and methods of the present invention provide temperature compensation for optical filters and other electronic equipment, such as interference filters and etalon filters, by adjusting the angle of incidence of an optical signal relative to the optical filter as the temperature of the apparatus changes. This adjustment to the angle of incidence shifts the center wavelength of the filter and offsets a countervailing center wavelength shift in the filter induced by the temperature change. According to one aspect of the invention, dense wavelength division multiplexed (DWDM) optical signal processing equipment may thus provide improved performance while operating over a range of temperatures.